In order for a terminal to connect to a mobile communication network, mutual authentication is required between the terminal and the mobile communication network. Through mutual authentication, the mobile communication service provider identifies whether terminals are legitimate subscribers and provides the legitimate terminals with services, and the terminal also connects to the mobile communication network with security and makes voice chat and data communication. In mutual authentication, a subscriber identifier and an encryption key are used. Terminals store subscriber identifiers and encryption keys in the Universal Integrated Circuit Card (UICC) physically separated from them. When mutual authentication is made between an authentication server of a mobile communication network and a UICC of a terminal, the terminal relays authentication messages between the server and the UICC. A UICC is used to identify a subscriber and contains information about communication service providing companies in the process of manufacture. When users have a plan to change an existing mobile communication network of one communication service providing company through which they have received communication services to a mobile communication network of another communication service providing company while keeping their same terminals, they need to use an additional UICC compatible to the other communication service providing company supporting the new mobile communication network.